Lightroom Classic: Keyword hierarchies not being broken up when synced to Lightroom CC

My workflow has, for several years, been to import my shoots with Photo Mechanic because it speeds up certain parts of the process. I have always done my keywording in PM with pipes as separators (no spaces, so (top level)|(sub level)|(etc)). The keyword hierarchies have always displayed properly in Classic CC, with just the bottom level of each hierarchy visible, but the entire hierarchy has always been visible in LR CC as a collection of separate, flat keywords (just as with exported shots).

However, my recent imports, while still looking the same in Classic CC, are now problematic in LR CC - the hierarchies apparently aren't being broken up on import, so in Lightroom CC, instead of seeing USA, CT, and New Haven as separate keywords, I'm now seeing USA|CT|New Haven.

Was a change made somewhere that is causing this? If so, how do I adjust for it so that the keywords get broken up properly in LR CC?

Keywords do not sync at all between classic and CC. The only reason you see these is that they were present in the original files which is a bit of an unsupported hack. I would completely ignore the keywords in cc and just focus on Classic. Adobe has stated that there will be no improvements on the syncing from Classic.

I'm sure there have been updates to PM over the last few months, but since I'm using XMP sidecars, there's no embedding involved.

I've just taken a look at a couple random XMP files, and they look like they're structured identically. Both have a "dc:subject" section which has one line for each flattened keyword, and a "lr:hierarchicalSubject" section which has the hierarchical keywords with the pipe separators.

If I was to guess, I think Adobe changed something at some point in 2017 (I'm seeing the change between images processed with LR CC 6.8 and 6.10.1, but both uploaded with Classic CC 7.2) and is now using the "lr:hierarchicalSubject" section instead of "dc:subject", which I would classify as a bug unless they can give me a reason for it, since we know that LR CC doesn't do hierarchical keywords.

Embedding and sidecars is the same thing in this respect. It’s possible that Adobe is considering adding hierarchical keywords for Lightroom CC at some point. Not to sync them with Lr Classic, but because there is a strong demand for it in Lr CC too.

I know that, which is why this is a mystery. Why is LR Classic CC uploading the entire hierarchical keyword string from the "lr:hierarchicalSubject" section of XMP to LR CC instead of the flattened "dc:subject" section?

As described in this thread https://feedback.photoshop.com/photoshop_family/topics/lightroom-classic-cc-keyword-hierarchies-not-... , it appears that Lightroom Classic CC is writing keywords that Lightroom CC is able to display after initial upload using the "lr:hierarchicalSubject" section of the XMP sidecar rather than the flattened "dc:subject" section. While this maintains the keyword hierarchy that Lightroom CC doesn't otherwise support, it leaves the keywords in ugly strings (top_level|sub1|sub2|sub3) rather than a set of separate keywords. This seems odd, since Lightroom CC does away with hierarchical keywords entirely, so why read from the XMP section that explicitly applies to keyword hierarchies?

This change seems to have happened over a year ago, but I didn't notice it because I was only using Lightroom CC for certain parts of my workflow where keywords weren't ever a consideration, and the large bulk of my library wasn't uploaded. It may relate to the change from Lightroom CC 6.x to 7.x - that's when the switch happens in the files I've looked at.